1. Field
The invention relates to an optical transmission device, an optical transmission system, and a bandwidth control method for transmitting client data encoded according to an 8B/10B encoding system, between a client side and an optical communication network side. In particular, the invention relates to an optical transmission device, an optical transmission system, and a bandwidth control method for equilibrating the bandwidth of client data and thereby avoiding an occurrence of frame discard on the destination client side, and transferring the client data transparently.
2. Description of the Related Art
In recent years, in optical transmission systems employing optical communication networks, optical transmission devices are used that demultiplex a plurality of client signals according to an 8B/10B encoding system and that transmit physical layer data in a framed form based on a frame format technique such as GFP (Generic Framing Procedure), SONET (Synchronous Optical NETwork), and SDH (Synchronous Digital Hierarchy) (see, for example, Patent Document 1).
FIG. 14 is a diagram showing an example of a typical optical transmission system. As shown in this figure, the optical transmission system is constructed, for example, such that a plurality of optical transmission devices are connected to each other in a ring shape via an optical communication network (Network) and client networks each containing a plurality of client devices are connected to each optical transmission device.
Here, client signals to be transmitted and received between the individual client devices are encoded according to an 8B/10B encoding system, then multiplexed by an optical transmission device, and then transmitted through the optical communication network. In such an optical transmission system, it is desired that in the optical transmission devices, the client signal can be transferred transparently in the intact form of physical layer data.
Specifically, requirements are that: (1) setup information is transparently transferred between client devices so that an optimal operation mode is set up between the client devices; (2) failure information is transparently transferred between client devices so that switching is performed normally at the time of occurrence of a failure; and (3) frames are transparently transferred so that even when header information or the like is used in accordance with a non-standard specification, a network is constructed without the necessity that the individual client devices should recognize the presence of optical transmission devices.
In a case that a client signal is to be transparently transferred to a counterpart client device in the intact form of physical layer data, when a client signal having a frequency deviation is transparently transferred, the frequency deviation easily causes underflow or overflow in a FIFO (First In First Out) buffer (simply referred to as “FIFO”, hereinafter) in a transmitting section of a client interface section.
Thus, in general, the optical transmission device performs “Rate Adaptation” in which an inter frame gap (Inter Frame GAP) is inserted into the signal to be transmitted to the client device or alternatively an IFG is deleted so that the rate is controlled.
Here, rate adaptation in a typical optical transmission system is described below with reference to FIGS. 15, 16, and 17. FIGS. 15, 16, and 17 are parts (1), (2), and (3) of a diagram describing rate adaptation in a typical optical transmission system. These figures show a case that WDM (Wavelength Division Multiplexing) transmission devices are employed as optical transmission devices.
For example, as shown in FIG. 15, an optical transmission system is described in which a plurality of WDM networks 30 (Network) each constructed from a plurality of WDM transmission devices (WDM SYSTEM Term) are connected in a tandem configuration, and in which a client network 20a is connected to a WDM transmission device 10a located at a starting point while a client network 20b is connected to a WDM transmission device 10b located at an ending point.
FIG. 16 shows the WDM transmission devices 10a and 10b shown in FIG. 15. Each of the WDM transmission devices 10a and 10b is an optical transmission device employing an 8B/10B encoding system. Thus, each device multiplexes client signals to be exchanged between client devices connected to the client networks 20a and 20b, and then transmits the signals through the WDM network 30.
As shown in FIG. 16, the WDM transmission device 10a has a client interface optical module section (Opt.MDL) 11a, a client interface section (ClientINF) 12a, a network interface section (NetworkINF) 13a, and a network interface optical module section (Opt.MDL) 14a. 
The client interface optical module section 11a converts a client signal exchanged between the client network 20a and the WDM transmission device 10a, from an optical signal into an electric signal or alternatively from an electric signal into an optical signal. The client interface section (ClientINF) 12a has a FIFO for accumulating client signals, and encodes the client signals according to an 8B/10B encoding system.
The network interface section 13a performs processing concerning the framing of a client signal like digital wrapping. The network interface optical module section 14a converts a client signal exchanged between the WDM network 30 and the WDM transmission device 10a, from an optical signal into an electric signal or alternatively from an electric signal into an optical signal.
Similarly, the WDM transmission device 10b has a client interface optical module section (Opt.MDL) 11b, a client interface section (ClientINF) 12b, a network interface section (NetworkINF) 13b, and a network interface optical module section (Opt.MDL) 14b. 
Further, transmission of a client signal performed by the WDM transmission devices 10a and 10b is described below in detail with reference to FIG. 17. For example, a case is described that a client signal is transferred from the client network 20a to the client network 20b. 
First, when a client signal is transmitted through the client network 20a, on the WDM transmission device 10a side, the client interface section 12a receives the client signal through the client interface optical module section 11a. 
Here, as shown in part (1) of the figure, the client signal (Client data) transmitted through the client network 20a contains a frame A (Frame A), a frame B (Frame B), and the like. Then, an inter frame gap (InterFrameGAP) is inserted between the individual frames.
When receiving the client signal, the client interface section 12a accumulates the client signal temporarily into the FIFO, then reads the signal sequentially, and then transfers the signal to the network interface section 13a. 
Here, as shown in part (2) of the figure, when the amount of data accumulated in the FIFO goes below or is at a predetermined threshold value, the client interface section 12a inserts a padding signal into the frame contained in the client signal, and then transfers the client signal to the network interface section 13a. This padding signal may be, for example, 65B_PAD of GFP-T (Generic Framing Procedure-Transparent) according to ITU-T (International Telecommunication Union-Telecommunication standardization sector) G.7041 standard.
When the client signal is transferred from the client interface section 12a, the network interface section 13a transmits the client signal through the network interface optical module section 14a to the WDM network 30.
On the other hand, when the client signal is transmitted through the WDM network 30, on the WDM transmission device 10b side, the network interface section 13b receives the client signal through the network interface optical module section 14b. When receiving the client signal, the network interface section 13b transfers the received client signal to the client interface section 12b. 
When the client signal is transferred from the network interface section 13b, as shown in part (3) of the figure, the client interface section 12b deletes the padding signal so as to restore the client signal, and then accumulates the restored client signal temporarily into the FIFO.
Here, as shown in part (4) of the figure, in order to prevent overflow or underflow in the FIFO, the client interface section 12b performs rate adaptation in which an inter frame gap is inserted into or deleted from the client signal. After the rate adaptation, the client interface section 12b transmits the client signal through the client interface optical module section 11b to the client network 20b. 
For example, when a network is constructed by employing a Gigabit Ethernet (registered trademark) (referred to as “GbE”, hereinafter) according to IEEE (Institute of Electrical and Electronics Engineers) 802.3 standard, a frequency deviation is ±100 ppm and an idle code (Idle Code) and a C1C2 code for auto-negotiation (C1C2 Code) correspond to an inter frame gap.    [Patent Document 1] Patent No. 3690516 (Japanese Laid-Open Patent Publication No. 2003-188843)
Nevertheless, when rate adaptation is performed, the bandwidth of the frame of the physical data transmitted to the client device fluctuates to an extent of 100% or greater relative to the frequency deviation. This causes a problem that when the optical network side frequency of the optical transmission device is higher than that of the client signal and hence the inter frame gap of the input signal reaches a minimum value having been set forth for each client device according to an 8B/10B encoding system, the processing load increases in the counterpart client device so that frame discard occurs. Then, the frame discard causes a problem that transparent transfer of a client signal in the intact form of physical layer data cannot be performed.
For example, in the case of a GbE employing an 8B/10B encoding system, the inter frame gap of the client signal is greater than or equal to 96 ns (12 bytes) after 8-bit conversion according to 8B/10B encoding. However, when a frequency deviation occurs, the inter frame gap may be reduced down to 64 ns (8 bytes) according to the definition of IEEE802.3 or ITU-T G.7041 standard.
Thus, between the client devices, even when the bandwidth is changed in a state that the length of the inter frame gap is reduced to a value near the minimum value of 64 ns (8 bytes), the client signal need be transparently transferred without the occurrence of frame discard.
In particular, as shown in FIG. 15, in the optical transmission system in which a plurality of optical communication networks each constructed from a plurality of WDM transmission devices are connected in a tandem configuration, when rate adaptation is performed by the individual WDM transmission devices, their frequency deviation causes a fluctuation of 100% or greater in the bandwidth of the frame of the physical layer data transmitted to a client device at far end (in GbE, the bandwidth becomes 100% when the inter frame gap is 96 ns).
At that time, the inter frame gap reaches a value near a minimum value (64 ns in the case of GbE) set forth for each client device according to an 8B/10B encoding system. A larger value of the number of tandem stages results in a stronger tendency that the frequency deviation between the individual devices causes a bandwidth of 100% or greater in the frame of the physical layer data between client devices at far ends.
As such, when the bandwidth is 100% or greater or alternatively its fluctuation is 100% or greater, in a client device that requires a large amount of time in the frame processing for the physical layer data, the processing load increases so that frame discard occurs. This causes a problem that transparent transfer of a client signal in the intact form of physical layer data cannot be performed.